Tool to check capacitance value

[Moonman]

I'd like to test the value of capacitors while they are in the PCB, to check if they need replacing.

Would this work?

LCR Meter Digital Capacitance Inductance Resistance Tester with Battery Crocodile Clip Bag LCR Tester Measuring Meter: Amazon.co.uk: Car & Motorbike

LCR Meter Digital Capacitance Inductance Resistance Tester with Battery Crocodile Clip Bag LCR Tester Measuring Meter: Amazon.co.uk: Car & Motorbike

www.amazon.co.uk

 

[Nigel Goodwin]

No, and it's a pointless exercise anyway - 'generally' only certain types of capacitor tend to fail, and mostly only in certain parts of the circuit. They also don't usually lose their value, but go high-ESR instead - removing them from circuit and testing them with that meter would show them as OK, even when they are completely useless.

With an ESR meter you can check electrolytics in-circuit, to a certain extent (and as a by-product they often display the value as well) - but it's the ESR that is important. If the value has gone low (which is fairly rare) then the ESR will have gone massively high.

 

[Moonman]

I've looked into this a little.

Would this ESR meter be useful for in-circuit measurement?

**broken link removed**

Since it's effectively only measuring the resistance of the capacitor, what's wrong with doing the same using a digital ohmeter?

 

[Les Jones]

A normal ohm meter which does it's testing using DC should read infinite resistance when connected to a capacitor.
As the reactance of a capacitor is 1/(2 x π x F x C) when F =0 (I.E DC) the reactance is infinite. ESR meters normally use a frequency of about 100 Khz for testing.

Les.

 

[Moonman]

Right, so a.c is required to measure capacitive reactance (ESR of capacitor).

 

[Nigel Goodwin]

Right, so a.c is required to measure capacitive reactance (ESR of capacitor).

Yes, and 100KHz is the standard frequency used. What capacitors (function in the circuit) are you wanting to check?, what in?, and why?.

 

[Moonman]

An old Betamax VCR that I'm trying to return to working order.

While it's apart, I'd like to check the caps and replace.

 

[JimB]

so a.c is required to measure capacitive reactance (ESR of capacitor).

Let us be quite clear, capacitive reactance is NOT the same thing as ESR (Equivalent Series Resistance).

Reactance is a normal property of a capacitor.

Capacitors also have a VERY SMALL resistance when operating normally.
When capacitors age that resistance increases. When that resistance is measured it manifests itself a a parasitic resistance in series with the capacitor, hence the name Equivalent Series Resistance.

JimB

 

[Nigel Goodwin]

An old Betamax VCR that I'm trying to return to working order.

While it's apart, I'd like to check the caps and replace.

If you want to do that, then replace them all - there's little in a VCR that would cause capacitors to fail high ESR - it's almost entirely a function of cheap capacitors used in switch-mode PSU's. Some of the later Betamax machines 'may' have used switch-mode supplies, but usually they were in separate screened metal cans.

Personally, I'd get the mechanics working before worrying about the electronics, as almost all faults were mechanical. We disposed of all of the VCR tools, spares and jigs ages ago now.